It is useful to know the location of people or objects for several reasons. The location in-and-of itself is important because it allows another party to find something that is lost, such as a child or a piece of expensive equipment. Location can also be valuable as a piece of data used in conjunction with other information. For example, knowledge about the location of a portable laptop computer combined with knowledge about the location of all the printers in a building allows a system to automatically route a print job from the laptop to the nearest printer, thus saving time and aggravation. The knowledge of who is in a particular room allows a system to adjust the temperature or lighting of that room to the individual's preferences or route that person's telephone calls to the phone in that room. These applications are presented here as examples illustrating the utility of systems that allows the location of a person or object to be known.
Existing systems suffer from several limitations. There is generally a direct tradeoff in these systems between precision and cost. If location needs to be known only to the resolution of a floor or a building, existing systems can be quite inexpensive. If however, location is needed down to the room level system costs are much higher, sometimes prohibitive. This is particularly true with systems that use RF and US techniques for location. For example, one such system from PinPoint costs about $6.84 per square foot of coverage to resolve location to a room. This is contrasted with IR-only systems, which are about half that cost at $3.89 per square foot (Based on Frost & Sullivan Report Number 5946-1: US Wireless Location/Tracking Markets, August 1999, pp 3-15 to 3-17.) Although IR systems are generally simpler and hence cheaper than RF or US systems, they have the fatal flaw of failing when the transmitters are obstructed. Therefore, the user is faced with either a very expensive system that is precise and reliable, or a less expensive system that fails to work some percentage of the time. Faced with these two options most users have decided to forgo the installation of location systems, despite the benefits such a system would provide.
Another limitation of systems that use IR signals is short battery life. Assuming that the location of a tagged object can be determined from one packet of data, the data contains 80 bits of information, is transmitted at a 1 KHz data rate, and enough current is passed through the LED to get a 20 to 30 foot range, one packet consumes approximately 135 mW of power. This means that for a reasonable size battery of 500 mAh capacity, and a transmission rate of once every 10 seconds, the user can expect a battery life of less than 58 days. RF transmissions are more power efficient (consuming about 50 times less power to send the same data) but have the undesirable characteristic of penetrating walls, which makes determining location based on them more difficult.